Recently, in accordance with improvements in image quality of liquid crystal display devices (LCD), employment of the LCD in place of the CRT (Braun tube) that is a popularized display is progressing in a wide variety of end uses. Accordingly, there is a demand for products that exhibit much higher quality image display performances in terms of a color reproduction range and brightness. Improvement in performances of the color filter is a key to responding to such the demand. This is because the color filter plays a role in coloring of display image of a LCD panel and the like, and therefore directly controls color properties of the LCD panel and remarkably influences the basic display properties.
As properties required for the color filter, high optical transmission, color purity, high contrast, lowered reflection, and the like can be mentioned. In particular, a low optical transmission and a low contrast cause a display screen to become dark owing to attenuation of light and causes discrimination between dark and bright to become unclear. Therefore, a high optical transmission is desired. Besides, it is also desired that a reproduction range of color that is obtained by combining R, G, and B is wide, and purity of each of thus-obtained color is high.
Under these circumstances, insofar as only improvement in transmittance of the color filter is required, this requirement can be fulfilled, e.g., by reduction in pigment particle size. However, the reduction in pigment particle size degrades the color purity and causes a narrow range of color reproduction. On the other hand, when a pigment concentration increases to widen a color reproduction range, a transmittance deteriorates. In other words, there is a tendency of these two requirements, i.e. the transmittance and the color reproduction range, to be contradictory to each other. Upon a further consideration of durability, it is absolutely impossible for the color filters currently in use to meet all of those requirements.
As described above, pigments have been used for color filters because each of them has a bright color tone, and high in coloring power and light resistance. For a practically important material among the pigments, in general, a fine particle form of pigments is frequently employed, i.e., the pigments are formed into fine-particles preventing them from aggregation so that bright their color tone and high coloring power can be obtained. In this regard, in many cases, when the fine pigment particles are reduced in size by a physical method such as salt milling, the dispersion of fine pigment particles results in a high viscosity liquid. Consequently, when the pigment dispersion liquid is prepared on an industrial scale, there arise the following problems: the pigment-dispersion liquid prepared can not be taken out of a dispersion machine, the dispersion liquid can not be transported through a pipeline, and further the dispersion liquid is solidified into a gel form during its storage.
To solve these problems and to obtain a pigment-dispersion liquid or colored photosensitive composition improved in fluidity and dispersibility, it has been attempted to modify the surface of an organic pigment (see, for example, JP-A-11-269401 (“JP-A” means unexamined published Japanese patent application) and JP-A-11-302553) and to use various dispersing agents (see, for example, JP-A-8-48890 and JP-A-2000-239554). Further, JP-A-2004-123853 and JP-A-2003-336001 describe reprecipitation of forming particles in which a sample dissolved in a good solvent is injected into a poor solvent.
When these pigments are used for the colored photosensitive composition, it is generally required that a layer formed by the colored photosensitive composition is extremely thin and that the layer shows a high coloring density even in such small thickness. Accordingly, e.g., an organic pigment is to be shaped in fine particles of highly and uniformly reduced in size and dispersed in an organic solvent. However, at present, there is no pigment and no color filter using the same satisfying those requirements.
As a further requirement for the color filter, there is a requirement for compatibility between cost reduction and contrast improvement. As one measure to reduction in cost, application of an inkjet method in forming RBG pixels is conceivable. On the other hand, as a contrast-improvement measure, size reduction of pigment particles is conceivable. However, as mentioned above, pigment particles currently in use cannot respond to those requirements. According to our further verification, pigment particles prepared by a traditional method, such as beads dispersion, cause deterioration in stability of ink used for forming pixels, and thereby ink-nozzle clogging occurs. Therefore, the ink cannot be manufactured with stability, so the ink productivity is lowered to result in a cost increase. In addition, unevenness of color in the color filter is occurred, and a decrease in color filter density is observed during the long-term use of a panel. Moreover, a color filter disclosed in JP-A-2003-33600 occurs dissolution of pixel materials and roughness of the color filter surface.